Conventional:
A pilot's input is being transfered to the surface via tons of cords, mixer units (e. g. for the Autopilot), Rolles and stuff. Usually there are 3 cords running parallel as a backup. Hydraulicly driver servos augment the inputs.

Fly-by-wire:
The pilot's input are being processed by computers, where tons of stuff is being done with the signal, and then wires lead to the servos. The control-surface-servos are still driven by the hydraulics

Prolly coming up in the next years:

Fly-by-optics:
Same as fly-by-wire but all the computer boxes are connected by a optical bus. saves weight and is more reliable.

Fly By Wire uses hydraulics. Control surfaces are moved by hydraulics, but commands from the control-column are carried to the control-surface via electrical signal. The electrical sygnal commands the hydraulic system to move the control surface. The hydraulic system still moves the control-surface.

Fly-by-Wire does not require digital computer's. Analog one's work just fine as well. The CF-105 Arrow, which was built in 1958, had Fly By Wire. It was the most advanced aircraft of its time. The control colum input is analog, the computers convert it into a digital signal. That goes to the control surfaces, then translates back to analog before reaching the actuators (I think). They call this Direct-mode. The A-320 has direct mode only for the stab, and the rudder. Nothing else.

All fly-by-wire is not the same. Even digital fly-by-wire isn't. There's unaugmented fly-by-wire. That is the computer simply does whatever the hell the pilot tells it to do. Augmented fly-by-wire is usually always digital in nature, but it makes small adjustments to the control-surfaces to stabilize the aircraft. Many aircraft that used fly-by-wire such as the F-16 are longitudinally unstable (it would cartwheel backwards in mid-air if the computer didn't make constant adjustments, CL is in front of the CG, which makes for very interesting results), the computer stabilizes it. Longitudinally unstable aircraft tend to be very agile. Additionally, airliners can profit from this system in turbulent weather, the elevators are constantly being adjusted to avoid the plane from getting bopped up and down. The L-1011 (I'm not certain if this is FBW or not, I don't think it is)-500 has active control ailerons which tie into accelerometers, which measure vertical movements. They counter it by deflecting the ailerons up in updrafts to keep the wings from being excessively deflected to ease the loading on the wings. FBW planes tend to be quite stable even in turbulent weather. The 777 doesn't require back elevator to make turns; simply turn the yoke, and it automatically kicks in the elevator. I think this only works up to 25-35 degrees though.

The real thing about FBW that gets all these Boeing vs. Airbus debates is called Performance Envelope Protection. It's a safety system so to speak, which is designed to keep the aircraft from stalling, overspeeding, over-g-ing, or from potentially going upside down. It's also used to prevent the aircraft from taking on excessive descent rates. There are two ways to do this, and this makes all the difference between Boeing and Airbus.

Boeing uses soft-limits, and Airbus uses hard-limits.

Soft Limits are limits that are programmed in by the manufacturer in an arbitrary fasion. When the captain or pilot flying attempts to exceed these limitations, the computer will resist the pilots inputs, in addition to sounding many annoying alarms and warnings. However the pilot possesses the urge to override this. This could be done with a switch in the cockpit, or in the 777's case, by a good hard throw of the wheel!

The 777 has a bank-limitation of 35-degrees, which can be overridden to 66-degrees. Once you attempt to exceed this, the computer automatically starts attepting to roll the aircraft the opposite way, by a 30-degree counter-wheel movement. IF the pilot holds the stick where it is, it would be felt as quite a bit of resistance. However he or she is capable of overriding it by simply overpowering the system. Not really recommended, but there are circumstances where this feature can be quite useful.

Hard-Limits are used by Airbus. Requires a more-complicated code. The airbus system has pre-programmed limits in it's system as well. But unlike Boeings, there is NO override to them. The computer makes the final call. This is what upsets many Boeing fans . Limits for the A-320 include a maximum pitch Angle of 30 degrees, maximum bank-angle of 67.5 degrees, maximum vertical-acceleration of 2.5 G's and -1 G's. It also will not allow the aircraft to descend past a certain altitude without the gear down, will not allow go-around mode to be engaged below 100 feet AGL, climb past a certain altitude if it senses problems in the pressurization system (including in-secure doors). The pilot could pull and pull on his big-ole stick all he wants, and the plane will not exceed these limits. No matter what.

Each one has their own advantages. The Boeing design relies on pilot competence, the fact that these guys are very well trained, have thousands of hours logged, and really don't want to end up dead! The Airbus design kind of relies on pilot incompetence and sort of acts as a computerized baby-sitter for them. It does have some safe-advantages in the fact that the aircraft simply can't be stalled, overspeed, overstressed, or rolled-inverted (unless they get a really nasty wake turbulence encounter ). Boeing's advantage is that the aircraft is protected, but in an emergency the captain or pilot can take the aircraft to it's absolute limits.

To be honest, I agree with the Boeing method. Having a PPL myself, I think that having spent quite a bit of money (particularly the fact that much of it was my family's money!) on a PPL, undergoing the training involved, I think that I'd actually like to fly the aircraft without being interrogated by a committee of computers. Why bother spending all that money learning how to fly if the computer's gonna do whatever it wants regardless what you do! I think the pilot should be in ultimate control. Simply put, computers are not yet advanced enough to be able to handle unique emergencies. Put a computer in control of Fl- 232 when it blew it's fan-disk and it would have kept rolling until it went onto it's back and dove straight into the ground probably going supersonic when it did it! Now put a human being behind it and it's still iffy, but Captain Haynes managed to do it. The fact that he had 33 years experience with United, and 4 years of Marine training (he was a Marine Corps flight instructor), nearly 30,000 hours of flight time logged, and 7,000 of that being in the DC-10, combined with an experienced crew, and a DC-10 captain named Dennis Fitch (check-captain), may have had just a *little* to do with it . Even when computers do get that advanced, I'd still like to be the one who makes the final call anyway. The 777 is also more backed-up in case of an electrical failure, with 3 of the flight spoilers and the inboard aileron still backed up via mechanical means, with stab-trim remaining, and manual-rudder control. Hey, it's NEVER EVER supposed to fail, but just in case, I still like that extra back-up it offers. Additionally, when the throttles make a power-adjustment, the throttles move, which I personally prefer.

Fly By Power: I think you mean Power by Wire. That is different in the way that the actuators themselves are electrical in nature. Not hydraulic.

Power By Wire replaces the hydraulic systems in an aircraft with electrohydrostatic actuators. Instead of having a centralized hydraulic system which moves control surfaces, EHA actuators are small pumps themselves. This system is lighter and has the potential to be more reliable.

> I think that I'd actually like to fly the aircraft without being interrogated by a committee of computers

Flying the Airbus is no different from flying any other plane (except maybe crosswind landings). In terms of flight controls, you pull back, the plane goes up. There's no obvious computer interaction there. The only place it becomes obvious is in an emergency.

Actually, you can take an Airbus to its limits more than you can in a Boeing. I suggest reading this http://www.airbus.com/pdfs/customer/fast23/p2to9.pdf
Sure, its written by Airbus, but it has good points.

NASA demonstrated on a MD-11 successful computer driven engine-thrust only landings, all with an outcome vastly different from Flight 232. Despite Haynes training, a computer did it far better. Computers react in miliseconds, can process far more data than a human, and never panic. Ever since the introduction of the A320, there has never been a use of the mechanical backup and the A380 won't have one.

It seems scary to trust your lives to a computer, yet we think nothing of trusting our lives to the buildings, cars, electrical systems and medicine various types of engineers design.

The only reason the MD-11 landing worked better than Haynes, was because it was programmed to perform the task. Such an event had already happened before.

Back in 1989 even if such technology existed, it had never happened, the computer would have never been programmed to handle such an event. Human beings can "make it up" as we go along. Computers only draw data from either an outside source (like an autoland) or from pre-written, pre-programmed data. A.I. is only in it's infancy.

So many situations where a computer would kill lots of people but a human being would somehow be able to scrape by.

Blackbird, I am impressed with your detailed knowledge of the flight envelope protection on the Airbus. I am sure I can find this myself, but perhaps you would care to describe what went wrong in the A320 that crashed during an airshow where the pilot made a "flight envelope-defying" low speed high aoa pass over the airfield, but could not initiate a go-around.

Jsuen, you say that since the intro. of the A320 there has never been a use of mechanical backup. What mechanical backup does the A320 have?

On the subject of EHA's, the A380 will have EHA's as emergency flight controls, with conventional hydraulic servo FBW as the primary controls. I am sure with the experience gained on the A380, EHA's will eventually migrate up the food chain to all flight controls -- probably 15-20 years out.

Blackbird,
Computers don't need to know every possible situation. Indeed, they can learn. Often, control is done through a feedback loop, where the computer adjusts itself to the responses of its inputs. For example, lets say a door falls off and creates additional drag. The computer doesn't need to know that the door fell off, but just that the airplane is yawing in one direction and losing airspeed. The computer will apply opposite rudder until it stops. By looking at the inputs and end results (black box model once again), we don't need to worry about the causes.

Now, at this point Airbus computers are not that complicated (NASA is working on the system described above). They simply apply hard limits to stalls, overspeeds, roll, and pitch limits. These situations, especially stalls, are most likely to occur quickly. Hard limits result in a desirable outcome for instinctive pilot actions (stick fully back, for example). If something happens and the plane can't turn right, the pilot is free to make left turns.

Also, the Airbus has a mode called Abnormal Law. That is, if the computer finds the plane in a situation exceeding normal limits (high pitch, for example), the plane will not use hard limits. The logic is that the computer wouldn't have let the plane into that situation, and therefore a malfunction has occured.

Manual backup is the horizontal stabilizer trim, rudder, and differential power. These are driven by hydraulics and cables, the conventional way.

The problem with the AF A320 was that due to poor planning and possibly misleading visual cues, the pilot flew too low, 30 instead of 100 feet. By the time the GA was initiated, they could not avoid hitting trees.

That is the analog signals from the control-columns bypass the main computers. There is no envelope protection in this case, but this is to only be activated in an emergency, such as the computers failing. It's still fly-by-wire. Just the analog bypasses the computers.

This system only covers the rudder, and the stab though. That's it. Ailerons, spoilers, elevators and all that stuff are useless.

Blackbird, I am impressed with your detailed knowledge of the flight envelope protection on the Airbus. I am sure I can find this myself, but perhaps you would care to describe what went wrong in the A320 that crashed during an airshow where the pilot made a "flight envelope-defying" low speed high aoa pass over the airfield, but could not initiate a go-around.

Well first off, few airline pilots in their right mind would attempt what Captain Asseline did at Habsheim. What Asseline did was Assenine! The reason they did was because they were told over and over again that the computer would protect them. Wouldn't allow them to stall, or do anything unorthodox. Despite this the A-320 is an ordinary plane by aerodynamics standard.

They came in, apparently they were going for the wrong runway at first , they made a quick adjustment. This did require them to stabilize their approach again. They came in over the field. Apparently the radar-altimeter malfunctioned and said they were at 100 feet when they actually were at less than 30. They activated TOGA by pulling full back on the stick, but it wouldn't activate since they were not 100 feet up. Asseline tried to firewall the throttles but the FADEC the airbus used would not allow this. Not to mention engines like those with the exception of military ones, take a couple of seconds (about 8 to 10) to spool up. They did reach about 95 N1 as they hit into the trees, but that was a bit too late.

Additionally below 100 feet, the computer will automatically assume the pilot is trying to land LOL. He kept pulling back on the stick, and the plane kept trying to lower it's nose, making asseline's attempts to pull up even worse! We all remmeber the rest when they nailed the trees and blew up. And you hear that french guy say "Oh no! Oh no!"

1] electrohydrostatic units were not introduced for fly by wire aircraft,as they were fitted to the British VC-10 jet liner designed in the early 1960,s, and if my memory serves me right it was a very good reliable system, especially from the redundancy position. They were controlled by fly by wire the old mechanical sort though.

2] If we are talking about electric fly by wire let us not forget the poor old Concorde, which has had this system fitted in a rudimentry way since the early 1970s. yes it has a mechanical back up but I cannot remember it being used in anger

Of course, there are always two sides to an argument. The soft-limits (Boeing) and hard-limits (Airbus) debate is not just a matter of limiting the pilot's controls or not. In an independant study on "terrain avoidance" in which the 777 and A330 were compared, it was concluded that the A330 which had hard limits were able to avoid terrain in a quicker, safer and more precise manner than the 777.

The idea behind the Airbus philosophy is that in a potentially hazardous situation, the pilot can simply yank the side-stick into the lower corner (full aft-stick and full bank.) The airplane will then pitch up to maximum pitch up altitude and maximum bank angle. At the same time, AlphaFloor kicks in the provide maximum engine power. This type of terrain avoidance manuver is only possible with Airbus' hard-limits without the risk of stalling the aircraft or bringing it out of its flight envelope.

The Airbus has three main laws, Normal, Alternate, and Direct. Normal enables full hard flight envelope protections. Full C* stick control is used where the stick commands roll rate and acceleration, turn coordination and yaw dampening are enabled.

Alternate law happens when multiple systems fail. Roll becomes direct-- sidestick movements are proportional to control surface deflection. Pitch remains in load factor mode. Low speed pitch down and high speed pitch up commands can be overridden.

Direct law is entered when even more systems fail. In this mode, sidestick movements are proportional to control surface movements.

There are "traditional" cable and hydraulic controls on the Airbus. It was necessary to win certification, as it was the first fly-by-wire aircraft. The rudder on the Airbus is mainly mechanical. Furthermore, note that the sidestick does not control stab and rudder. Perhaps somebody who flies/works on the Airbus can attest to this fact.

Jsuen: The Airbus has three main laws, Normal, Alternate, and Direct. Normal enables full hard flight envelope protections. Full C* stick control is used where the stick commands roll rate and acceleration, turn coordination and yaw dampening are enabled.

As far as I know, normal law is sequenced in at takeoff and out at touchdown (separately for the individual control surfaces); On the ground, the control surfaces are in direct mode. (With no or low airflow, the g-force requests from the sidestick can never be satisfied.)

VC-10, I am intrigued by your comment that the VC-10 had EHA's. It was certainly a beautiful and advanced aircraft.

What did you mean by They were controlled by fly by wire the old mechanical sort though? Which functions were EHA's, and were they primary or back-up, and how exactly did the electrical signal pass from the yoke to the EHA? Also, what electrical control technology was used to drive the hydraulic pump - variable speed/bidirectional, etc?

I worked on some experimental EHA's on military applications in the past 10 years or so, mostly on the hydraulic pump component.

...one more thing ... thanks Blackbird and Jsuen for shedding some light on the A320 accident at the airshow. I do recall, now that you have reminded me, what the problem was. I felt at the time that the computer had "too much" control and did not allow the pilot to override it. I guess the problem was also more to do with the shifting paradigm; as pilots become more familiar with the new control philosophy, they will be able to better adapt to such situations. While there were a number of control-related incidents early in the A320 service history, there has not been any in a good while.

delta-flyer,
It is going back some 26 years since i was involved with the VC-10 , but I will try my best to answer your questions
The VC10 had 4 aileron sections , 4 elevator sections , and two rudder sections. Each section was controlled by its own electro hydraulic actuator , which consisted of an electric motor, it's own individual hydraulic system, a hyd actuator and the mechanical control valve. The various motors were powered from seperate bus bars, which were normally connected ,but which could be split if required. In case of a complete alternator failure there was a Elrat which could be lowered to give electrical power to a limited number of EHa or as they were called on the VC-10 PCU. Other than the hyd driven tailplane and spoilers, these PCU were the only means of inflight control on the VC-10.
The conrol to them was by old fashioned cable runs , which gave control of the actuators in both directions but if my memory serves me correcrly movement was at a set not variable rate.
This was in my mind a great system as it gave great redundacy, and put the power where it was needed at the control surface direct

FBW will not work without power. The A320, 330 and 340 have manual controls for certification reasons and have never been used. However, there are redundant power sources (batteries, APU, IDGs, RAT) and independent power systems to make failure an impossibility.

Current FBW planes still rely on centralized hydraulics. which are more likely to fail than electrical power. Electrical power can be easily controlled with circuit breakers. Hydraulics require priority valves and inherently rely on mechanical components. This is one of the reasons why power-by-wire is favorable.

VC10, thanks for your reply. I see, now, the control was by means of the mechanical cables and hydromechanical servo-actuators -- only the hydraulic power generation was not central, but distributed near the actuators.

Today's EHAs are characterized by having the electric motor, pump, reservoir and actuator integrated into a single package. The pump ports are directly connected to the actuator extend/retract volumes. There is no control valving - the speed and direction of the actuator is governed by the speed and direction of rotation of the motorpump.

What is also being studied is to replace the control surfaces by piezo-elements, thus eliminating the need for hydraulics system to power them at all. You just have to apply a current to the piezo-element, and it moves. However, that still seems to be at least 10 years from airframe application.

Blackbird, are you sure about the 'no go around below 100ft' protection? I'm sorry, but I think you are incorrect about this particular feature. I have been in go-arounds on A330 and A319 aircraft where we have initiated a go around almost at the flare.

Incidentally, the Habshiem crash involved what was still a prototype aircraft (albeit in AF colours) and changes were subsequently made to the flight control software. In a current AI aircraft, if you put the throttles in T/o Go around power, it will give you go-around power (after spooling up of course!).